1. Field of the Invention
The present invention relates to rotary actuators and to the method of manufacturing rotary actuators. More particularly, the invention relates to an improved toroidal arc segment piston for a rotary actuator, to an improved method of forming such a piston, and to an improved method of forming an arcuate chamber in the rotary actuator housing for receiving the piston.
2. Description of the Background
Rotary actuators have previously been proposed to rotate a shaft about a fixed axis. Hydraulic or pneumatic fluid pressure may be applied to a rotary actuator to displace a piston within a chamber in the actuator housing and thereby directly effect rotation of the shaft connected to the piston. In a typical application, a rotary actuator has been proposed to open or close a butterfly valve. The valves may be installed in remote locations, and accordingly high reliability for the fluid pressure responsive actuator is required.
Almost all actuators used to rotate a valve stem and thereby control flow of fluid through a valve employ a piston which is linearly movable within the actuator housing. Such actuators may employ a rack and pinion mechanism, a scotch yoke assembly, a crank arm mechanism, or a hydraulic gear to convert the linear piston motion to rotary movement of a shaft. These motion conversion mechanisms represent a significant portion of the cost, weight and size of a rotary actuator, but are considered necessary to achieve the desired rotary operation based or linear movement of the fluid responsive piston.
A rotary actuator has previously been proposed which comprises a housing defining a chamber having a generally toroidal arc segment configuration. A similarly-shaped piston or torus oscillates within the housing chamber, and an arm structurally connects the piston and a shaft journaled within the housing. Fluid pressure is used to displace the piston within the housing and rotates the shaft in one direction, while either a spring or fluid pressure acting on an opposing end of the piston may be used to effect rotation in the opposite direction. A seal is provided to maintain fluid-tight engagement between the piston and the housing during piston reciprocation, and may be mounted on either the piston or the housing.
Those skilled in the actuator art have long appreciated the significant problems and high costs associated with manufacturing direct-acting rotary actuators which utilize rotary rather than linear piston movement, and with obtaining a reliable seal between the toroidal-shaped piston and the interior walls of the housing which define the receiving cavity for the piston. To reduce manufacturing costs, the housing is conventionally of a split body design, so that housing half sections are joined together along a plane which is perpendicular to the actuator shaft and passes through and splits the toroidal arc segment chamber into similar halves. If the seal is provided on the piston, the seal continuously engages this split housing seam, which greatly reduces seal life and actuator reliability. Accordingly, the housing/piston seal is often provided within a slot in the housing which encircles the chamber. In this case, the seal is thus stationary on the housing and engages the reciprocating rotary piston.
The problem is further compounded, however, because of the high costs associated with manufacturing a toroidal arc segment piston within the tolerances necessary to maintain fluid tight engagement with the stationary seal. In order to satisfy strength and shock resistant requirements, the rotary piston is preferably fabricated from metal. The surfaces on the piston need to be concentric with respect to the axis of the actuator shaft, and complicated and expensive NC machining techniques are generally proposed to provide dimensions within the desired tolerances. Moreover, the desired tolerances between the shaft and the piston sealing surfaces are affected by the attachment of the arm to the piston. Securing the arm to the piston prior to final machining of the piston sealing surfaces further complicates the machining operation. Alternatively, the machining tolerances for the arm may be closely controlled, and the variance or play introduced by the arm to piston attachment technique considered so that the rotary piston may reliably seal with the actuator housing.
One technique for reducing the cost of manufacturing a toroidal arc segment piston for a rotary actuator is disclosed in the U.S. Pat. No. 4,817,213. Improved techniques are required, however, to further reduce the cost of manufacturing a rotary piston, and to maintain a reliable seal between the piston and the actuator housing. The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for efficiently manufacturing a rotary actuator, and for obtaining a highly reliable seal between the actuator housing and the rotary piston.